Nuclear Reactions Li 3 6 Protons and neutrons are held together in nucleus by the strong force. The strong force is strong enough to cancel out the electrical repulsion between protons in small nuclei.

Strong force p/p repulsion Net force Strong force P/p repulsion stronger Net force weaker small nucleus large nucleus

As the number of neutrons in an isotope increases, the stability of the nucleus decreases due to a decrease in the strong force that holds the nucleus together. Normal hydrogen and deuterium are not radioactive, while tritium is. H 1 1 hydrogen H 2 1 deuterium H 3 1 tritium Isotopes: Atoms of the same element (same atomic number=same number of protons), but different numbers of neutrons (  different mass numbers).

Atomic mass: the weighted average of all of the mass numbers for an element’s isotopes. Just in: a new element, Beakerium, has been discovered in a local high school laboratory. 2/5 of every sample mass number of 50 g 3/5 of every sample mass number of 100 g Isotope 1Isotope 2 Q: What is the average mass of a sample of Beakerium? Atomic mass = (2/5)(50 g) + (3/5)(100 g)= 80 g

Nuclear decay

The most common types of radiation given off by unstable nuclei are: Alpha particles: helium nuclei He 4 2 Beta particles: electron emitted FROM THE NUCLEUS (are 10x more penetrating than  ) Gamma rays: high frequency electromagnetic radiation (are 1000x more penetrating than  ) e 0 or  or  

Example of alpha particle decay: Po Pb He Notice that there are 84 protons on both sides of the arrow. The Law of Conservation of Mass strikes again!! Transmutation: the changing of one element into another. When Po decays to Pb by alpha particle emission, it has transmutated. *decay = giving off a particle

Half-life: the amount of time required for half of the unstable element to decay (transmutate) into another element Time (s)Red (g)Green (g) What is the half life of the red element? 2 s

Time after starting the experiment (hours) Activity (Disintegrations per second) x x x

x x To determine half-life, calculate half of the original activity. 400 /2 = 200cps Use the graph to find the time required to drop to this value. The half life is 15 hours. x

A substance’s half-life is constant, regardless of the size of your sample. x x x x It takes 15 hours to go from: 400 cps  200 cps 200 cps  100 cps 100 cps  50 cps x

Hi, I’m Lenny! The other Isotopes say I’m a little unstable

Radioactive M&Ms!!! Mission 1: To determine the atomic mass of Candium. Caution: Failure to follow the directions by eating the M&Ms BEFORE you have completed both missions will cause your lab grade to self- destruct! Mission 2: To determine the half-life of Candium.

Radioactive M&Ms!!! Mission 1: To determine the atomic mass of Candium. Caution: Failure to follow the directions by eating the M&Ms BEFORE you have completed both missions will cause your lab grade to self- destruct! Mission 2: To determine the half-life of Candium.

I Xe e 0 + Ex: I-131 undergoes beta decay: Ex: Ba-137 undergoes electron capture: Ba e + Cs Ex: Np-237 undergoes two beta decays Np Am e 0 + e 0 +

Nuclear F sUnsUn i u ssionvs. Nuclear F sion Sm shing  h ger  U + Te Zr H + He H Spl tting  t nier

Half-life calculations Recall: for every half-life that passes, half of the radioactive element transmutates into a different element. Ex: Element X has a half-life of 11 days. How many grams are left after 33 days if you start out with 80 g? 1. Determine the number of half lives that have passed #half-lives = total time/time for one half life = 33 days/11 days= 3 half lives 2. Multiply the starting amount by ½ three times to find out how much is left over. Amt remaining = 80(0.5)(0.5)(0.5)=80(0.5) 3 = 10 g

What if there isn’t an even number of half-lives involved? A radioactive element has a half-life of 5 minutes. How much of a 10 g sample is left after 17 minutes? # half-lives = total time/half-life = 17 min/5 min= 3.4 half-lives Amount remaining = 10 g(0.5) 3.4 =0.95 g